1. FIELD OF THE INVENTION
The present invention relates in general to a fire detection and location system, and in particular, to a fiber optic fire detection and location system employing optical time domain reflectometry.
2. DESCRIPTION OF THE RELATED ART
Fire detection and monitoring have historically been difficult in many compartmentalized structures primarily because of the limitations of currently available sensors. These sensors are generally discrete in nature, requiring the use of large number of units for adequate coverages of a structure such as a naval vessel or other large complicated structures like offshore drilling rigs, power plants, petrochemical plants, warehouses, factories and buildings. Networking of the several sensors both to trigger local alarms and to notify a central monitoring point requires an expensive, custom-engineered system. Traditional fire detection sensors are often unreliable due to the misidentification of ambient conditions as fire.
As an example, smoke detectors are often used for shipboard applications. Opacity-type detectors are used in place of ionization-type detectors to limit radiation dose to ship's personnel and to avoid false alarms caused by background radiation. Opacity sensors, however, experience frequent false indications due to airborne substances including moisture and are often ignored as a result. In addition, one or more detectors are required for each of the several hundred compartments on board ship for adequate monitoring. Often, there are several spaces such as cable trays and electronics cabinets which are impossible to access with smoke detectors.
On some vessels, once a shipboard fire is correctly identified, the ship's personnel must communicate fire and damage reports to command by sound-powered phone or by messenger since these detectors alarm only locally. This type of situation can result in mistaken information and response particularly in view of the stress involved.
There exist techniques for interpreting Rayleigh back scattering along an optical fiber. A portion of the light traveling through an optical fiber is continuously backscattered due to Rayleigh scattering. Disturbances along the fiber length, such as changes in temperature and stress on the fiber have been shown to change the index of refraction and the proportion of backscattered light. The York G2 system employs this principle in conjunction with complicated electronics to provide a distributed measurement of the temperature profile along a fiber length.
U.S. Pat. No. 4,839,527 discloses a fiber optic sensor system for the detection and/or analysis of smoke, gas, or the like. This is achieved with discrete sensors at fixed locations.
U.S. Pat. No. 4,505,542 describes a fiber optic temperature monitoring system with a discussion of the different claddings available for fiber optics. While this reference teaches of the detection of fire, it does not provide for locating the fire or its progression since fire anywhere along the length of the waveguide will similarly interrupt the transmission of light.
Thus, there is a need for a simple, reliable fiber optic fire detection and tracking system capable for use in large complicated structures. U.S. Pat. No. 4,843,234 demonstrated that the length of an optical fiber can be determined from the transit time for light traveling to the free end of the fiber using an optical time domain reflectometer (OTDR). However, this reference is very particularly directed to the measurement of the tip location of a consumable electrode.